The construction field makes use of elongated steel elements in the form of rods, bars, wire and cable for applying a prestress in a structural element or for a capacity to take up a continuous tensile stress applied thereto and it is with such elements, whether they are used in prestressed reinforced concrete or simply for tensioning members for bridges and other suspended structures, that the present invention is concerned.
Conventional steel pretensioning or prestressing elements are composed of steel alloys with approximately the following composition:
0.7 to 0.9% by weight carbon PA1 0.1 to 0.3% by weight silicon PA1 0.5 to 0.9% by weight manganese PA1 balance iron and unavoidable impurities and associated elements which do not affect the properties of the composition. PA1 0.7 to 1.0% by weight Mn PA1 0.7 to 2.2% by weight Cr PA1 0.3 to 0.6% by weight Mo PA1 0.5 to 2.2% by weight Ni PA1 up to 0.45% by weight C (preferably at least 0.1% by weight C) PA1 balance iron and unavoidable steel impurities.
Such steels are used as concrete reinforcing steels in accordance with the German industrial standard DIN 4227, for example, having a tensile strength in the range of 1000 N/mm.sup.2 to 1600 N/mm.sup.2. The yield point is generally about 15% lower and both tensile strength and yield point, as well as elongation to break and the necking in characteristics at break generally have been found to be lacking as will be discussed below. For example, the elongation to break is usually a maximum of 7%, but generally is well below this level. The conventional pretensioning elements composed of such steels conforming to these standards have been found to be unusually sensitive to handling and emplacement operations and to anticorrosion techniques which have been used, and to have problems with respect to welding.
The indicated tensile strength is generated primarily by a cold working using cold rolling and stretching or drawing and these techniques are used, for example, in the fabrication of so-called ribbed re-bar, i.e. concrete reinforcing bars or rods which are rolled with ribs intended to promote gripping by the concrete.
The cold working and the alloy composition appear to result in a product which cannot be welded without serious loss of tensile strength and other properties, especially at the building location at which the element is to be used without pretreatment or extremely expensive techniques for providing elements of more weldable materials between the less weldable or more sensitive reinforcing elements. Finally we may note that these elements have very poor corrosion resistance.